This invention relates generally to an adaptive threshold circuit and, more particularly, to an adaptive threshold circuit for use with magnetic or variable reluctance sensors that produce an alternating voltage in response to rotation of a wheel.
Magnetic sensors are used in automotive applications to provide timing signals for the determination of, for example, engine crankshaft position and speed, and in anti-lock brake systems for determining wheel speed. This type of sensor is located adjacent to a driven wheel that has circumferentially spaced slots or teeth. The wheel is associated with a magnetic pick-up that includes a pick-up coil and a permanent magnet. As the wheel rotates relative to the pick-up, an alternating voltage is generated in the coil.
A disadvantage of a magnetic type of sensor is that the peak voltages generated are generally proportional to the speed of rotation of the wheel. Thus, the peak voltage generated can vary, for example, from 250 millivolts (mvolts) at low speeds, to over 160 volts at higher speeds. Some sensors are rated up to 200 volts. In order to correctly decode the generated signal, the receiving circuit must adapt the threshold voltage it uses to recognize a positive or high voltage level.
As mentioned, the voltages generated in the pick-up coil are generated when a slot on the wheel moves past the sensor. Any dirt or scratches on the surface of the wheel will generate output noise that is also proportional to the speed of the wheel. At high speeds, this noise can be several volts in amplitude.
As one possible solution to the noise problem, it has been proposed to utilize an analog circuit that has adaptive threshold control. One example of such a circuit is shown in the U.S. Pat. No. issued to Christenson et al., 5,144,233, granted on Sep. 1, 1992. However, analog adaptive controls have some drawbacks. First of all, they generally require an external capacitor to store the voltage for the next threshold. The charge must be stored for several milliseconds, which requires a sizeable capacitor to maintain accuracy. Second, unwanted noise spikes in the system can modify the charge stored on the capacitor. This in turn causes inaccuracies with the next input switchpoint. Third, since the circuit is mostly an analog circuit, processing variations will tend to cause large parametric changes in operation of the circuit.
The above-referenced related application entitled "Adaptive Threshold Circuit" to which this invention shares the same inventor and assignee is directed to solving the above-cited drawbacks. In doing so, the above cited approach employs a digital to analog converter coupled to a pulse counter which is made up of a number of interconnected flip-flops for performing a sequential counting operation. The interconnected flip-flops produce a count which is a function of the peak voltage detected. The converter then produces an output that is a function of the count magnitude in the counter and which is used to provide a threshold voltage. The threshold voltage is then compared with a voltage representative of the sensor output to achieve a square wave voltage.
As previously mentioned, peak voltages are generated from the magnetic sensor that can vary from millivolts at low speeds to hundreds of volts at high speeds. During constant speeds, the time between input pulses is generally fixed and the adaptive threshold circuit may accommodate the input threshold voltage. However, in order to accommodate sudden changes in amplitude such as those associated with quick decelerations, it may be desirable to modify the threshold voltage when changes in speed are expected, so that sudden changes in wheel speeds do not go undetected.